The current state of the art in the verification of shielding effectiveness is centered around the measurement of the transfer impedance of a shielded wire. Transfer impedance is difficult and inconvenient to measure because it requires access to the core wire of the circuit to be tested. Gaining access to the core wire requires that connectors be disconnected.
There is no real industry standard definition for electromagnetic shielding effectiveness of wiring. One commonly used definition for shielding effectiveness is the ratio of the current that would be induced on a wire without shielding to the current induced on the wire with shielding in place. When the shield is in place, assuming that the shield circuit has a much lower impedance than the core wire circuit, virtually the entire amount of current that would have been induced on the core wire is induced on the shield instead. Therefore, the Shielding Effectiveness (SE) can be approximated by: ##EQU1## From FIG. 2 it can be seen that: ##EQU2## Where V.sub.c is the potential developed along the core wire circuit. The core voltage V.sub.c is related to the shield current I.sub.shield by the transfer impedance (Z.sub.t) of the shielded wire. ##EQU3##
Shielding effectiveness is then the ratio of the impedance of the core wire circuit and the transfer impedance. From the expression above, it is obvious that the shielding effectiveness is not dependent only on the cable structure alone. The shielding effectiveness is a function of the load devices attached to the core wire. The transfer impedance is a characteristic of the wire alone. It should be noted that the Shielding Effectiveness is inversely proportional to the transfer impedance. Therefore, greater shielding effectiveness results from lower shielded wire transfer impedances.
Shielding effectiveness cannot typically be directly measured. It is often desirable to make an assessment of shielding integrity which is independent of what the cable is actually used for in each situation. (For example, Z.sub.device1 may change over time and hence the shielding effectiveness for that particular circuit will change, as well.) This can be done by concentrating on the transfer impedance. Note from the equation above that if the transfer impedance is known, the shielding effectiveness of a given circuit can be computed for any arbitrary load impedances. If a change in the transfer impedance is detected, then the effects of this change on the shielding effectiveness can easily be inferred. For this reason, assessment of shielding effectiveness usually involves a measurement of the transfer impedance of shielded wires.
A typical transfer impedance measurement is shown in FIG. 3. The core wire is connected to the shield at one end and both are connected to the aircraft structure. The other end of the shielded core wire is left open circuit. A known (measured) current is induced on the shield. The shield should remain at approximately 0 volts potential with respect to the ground structure which acts as the current return. The core wire voltage is measured relative to the ground structure at the open circuit end of the core wire. Then the transfer impedance is computed by: ##EQU4##
For a typical shielded wire, at frequencies below the cable resonance, the transfer impedance can be characterized by two elements: EQU Z.sub.t =R.sub.t +j.omega.L.sub.t ( 6)
where
R.sub.t -DC Resistance of the shield circuit PA1 L.sub.t -Mutual Inductance between the core wire and the shield.
There is a long history of making transfer impedance measurements in this way. The literature contains many examples of independent researchers who model shielded wires in this way as a means to assess shielding effectiveness..sup.1,2,3,4,5 Researchers in the field have developed models for predicting the transfer impedance of shielded wires based on physical attributes of the cable and shielding.